As a seat for a vehicle, seats which have a cushion material formed of polyurethane foam (below referred to as urethane) have been known. For the cushion structure of such a vehicle seat, a structure has been widely employed in which a cushion material made of urethane is mounted on a spring material, such as Contour Mat (trade name) or the like, or a plate or the like provided at a sitting portion frame, a back portion frame or the like, and these are covered in a fabric material.
Consequently, it has been known that the shape (a design form) and resilience characteristics of the cushion material and compression characteristics of the polyurethane foam greatly affect characteristics of body pressure distribution from a sitter, vibration absorption and suchlike. By laminating urethanes having various characteristics to structure a cushion material, it is possible to obtain a cushion material provided with spring characteristics (resilience characteristics) close to the spring characteristics of muscles of a buttock portion and the like of a sitter. However, with such structures, there have been problems in that the restoring force tends to be insufficient or that the weight thereof is heavy.
Accordingly, seats have been considered (see, for example, patent reference 1 to patent reference 4) in which, as a cushion material to replace urethane, a pair of ground fabrics and a three-dimensional woven fabric or two-dimensional woven fabric, which is formed by connective fibers arranged between ground fabrics, are stretched onto a back portion frame to constitute a cushion structure. Such a cushion material formed with a three-dimensional woven fabric or two-dimensional woven fabric is an elastic structure constituted with elastic fibers which are resistant to permanent set in fatigue, has a thinner form than urethane, and exhibits resilience characteristics as a substitute for urethane.
In a seat structure of the above-mentioned vehicle seat or the like, it is desirable to ameliorate fatigue caused by a long period of sitting. Here, ameliorating fatigue does not mean dulling of the biological reactions to fatigue, which are signals to the brain of the sitter demanding rest, but rather suppressing the accumulation of fatigue in accordance with sitting and, particularly, the rapid onset of fatigue. Principal characteristics of a seat structure that affect fatigue include three characteristics: body pressure distribution, posture maintenance and vibration absorption. Now, the characteristics of body pressure distribution and posture maintenance affect stresses on the sitter which are based on pressure forces, restraint forces and the like that act on the sitter, and these stresses principally affect peripheral fatigue. On the other hand, vibration absorption characteristics have an influence on stress based on the acceleration of vibrations transmitted to the sitter, and this stress principally affects peripheral fatigue in conditions when the whole body is exposed to vibrations.
Further, peripheral fatigue of a sitter is ameliorated by fluctuating movements. Such fluctuations are biological rhythm oscillations and are ordinarily non-linear oscillations. Stress as described above is ameliorated by fluctuations in blood flow, body movements and the like which are based on these biological rhythms, and peripheral fatigue is ameliorated. Therefore, at a seat structure, it is desirable to allow (not hinder) fluctuations of a sitter, and to more proactively apply stimuli to the sitter which match patterns of fluctuating movement of the sitter.
However, conventional seats as are described above have simply been formed so as to reduce body pressure at particular regions of a sitter and improve absorption of vibrations, impacts and the like, and consideration has not been given to ameliorating fatigue on the basis of the mechanisms of fatigue.    Patent reference 1: The specification of U.S. Pat. No. 5,013,089    Patent reference 2: Japanese Patent Application Laid-open (JP-A) No. 2002-177099    Patent reference 3: JP-A No. 2002-219985    Patent reference 4: JP-A No. 2003-182427